Conductivity coherence factors in superconductors

In all textbooks, the microscopic mechanism of superconductivity is described as an attractive interaction between pairs of electrons which are bound together by lattice polarization forces. The recent discovery of materials which superconduct at unexpectedly high temperatures, raised some doubts on the general validity of this picture. The author's prospect was to study their electrodynamic excitation spectrum in the microwave frequency range (0.1 to 10 cm-1), where the photon energy is of the same scale as the attractive interaction responsible for the pairing mechanism. In particular the author was interested by coherence effect, a characteristic peak that appears in the temperature dependence of the optical conductivity and is a consequences of the peculiar pairing symmetry. By developing a novel detection scheme, the author has measured for the first time the conductivity coherence peak on conventional superconductors: Nb and Pb. Then, the study is broadened to new classes of two-dimensional compounds including organic metals and the cuprates. For all those materials, the inferred pair symmetry was found to be invariant under time reversal, in full agreement with that proposed in the original model